Add docstring validation script (from pandas) (#238)

* First version of the script with no pandas stuff (some tests will need to be removed)

* Updating tests

* Removing the part of the validation that gets the public API objects, and receiving them as a parameter

* Updating tests

* Fixed bug making examples fail when pytest was called in verbose mode

* Replacing pandas by a stdlib module to pass tests when pandas is not available

* Making code py2 compatible

* Simplified script (just one validate function + Docstring), and removed concept of warning

* Fixing imports in py2

* Changing import to see if py2 is happy

* Restoring imports, and calling pytest as a module

* Getting new changes from pandas sprint, and removing py2 stuff

* Fixing import error in tests

* Adding tests and removing unused code (improving coverage)

* Better implementation of module import based on code review

* Remove running examples

* Require first letter to be upper case only if it's a letter

* Allow one liner docstrings with quotes in the same line.